ROBERT W. WITTE and ROBERT L. MC DONALD
STANDARD MISSILE: GUIDANCE SYSTEM DEVELOPMENT
To achieve a missile capability that is commensurate with the long-range, high data-rate, target detection characteristics of the AN/ SPY-IA radar, an evolutionary upgrade of the existing STANDARD Missile-l was authorized. Designated STANDARD Missile-2, it adds a command mid course flight phase, thereby providing the AEGIS Combat System with increased intercept range and higher firepower . STANDARD Missile-2 with inertial midcourse guidance also provides upgraded TERRIER and TARTAR Combat Systems with a corresponding increase in capability. Terminal guidance accuracy has also been improved by incorporating a new homing receiver common to all STANDARD Missile-2 variants.
INTRODUCTION The design evolved over a two-year period, during which frequent meetings were held to review every The development of STANDARD Missile-2 (SM-2) aspect of the signal processing and logic. During that began in the early 1970's, with General Dynamics/ time, the communication links had progressed to the Pomona as the Design Agent and APL as Technical point where hardware interface tests were being con Advisor. At the time, STANDARD Missile-l (SM-l) ducted to demonstrate ship system compatibility. was in production as the primary weapon for Fleet Prior to the beginning of flight testing at White air defense by TERRIER and TARTAR ships. The Sands Missile Range in 1975, a guidance section principal modification to SM-l envisioned at the out design model was supplied to APL for an assessment set was to add a midcourse guidance system consist of its performance. Hundreds of hours of tests were ing of communication links, a digital guidance com conducted in the Guidance System Evaluation Labo puter, and an inertial reference unit. These additions ratory. As that evaluation proceeded, SM-2 began to were incorporated as shown in Fig. 1. For terminal acquire a reputation as a highly accurate missile. The guidance, the original plan had been to use the exist results obtained in the APL tests were confirmed in ing SM-l scanning receiver. However, shortly after subsequent flight tests of SM-2 and an upgraded SM-l becoming the Naval Sea Systems Command AEGIS that also uses the new homing receiver. To date, both Project Manager in 1972, Rear Admiral W. E. Meyer missiles have achieved a sizable percentage of direct made the decision to include a new monopulse ter hits in intercepting target drones. minal homing receiver. Thus, the final SM-2 configu The development and deployment of SM-2 cannot ration consisted of a largely new guidance section, a realistically be a stopping point in providing the fu repackaged autopilot to accommodate the inertial ture Navy with an effective antiair warfare capabil reference unit, and the existing SM-l ordnance, pro ity. Because the threat has continued to increase, the pulsion, and steering control systems. initial SM-2, now designated Block I, is being im To accelerate the development of the new homing proved. Currently, Block II is in engineering develop receiver, General Dynamics/ Pomona and APL engi ment, with APL technical support being provided in neers embarked on an intensive cooperative effort. many areas, including guidance.
SM -2 UPGRADE Figure 1 - STANDARD Missile-1 Communication linkS/Digital guidance computer (SM-1) showing STANDARD Missile- Monopulse receiver I / Inertial reference unit 2 (SM-2) additions. Evolutionary im provements to STANDARD Missile have been facilitated by the use of modular packaging. The SM-2 up grade consisted of a new guid -~ ance section and additions to the fmprncrf:---; autopilot/battery section, while R,dome I Gu;d,noe \ Au,op;lotlb'tte", Ro,ke'mo'o' S'eedng oon' Vo lullle 2, N Ulllber4, 1981 289 MISSILE GUIDANCE TECHNIQUES either target-reflected energy from shipboard illu mination or jamming energy emanating from the tar Semiactive Guidance get. The seeker consists of a gimballed antenna and a A feature common to both SM-l and SM~2 is an radar receiver that measures the angle of arrival of ability to "home," i.e., to guide either some or all of the received signal (Fig. 2). The seeker tracks the tar the flight by means of target-reflected energy. The get and provides information to the missile guidance source of this energy is a high power, continuous computer concerning the line-of-sight angle and the wave, shipboard radar illuminator. A portion of the closing velocity as determined from the Doppler fre illuminator signal is received directly by the missile quency. Using a proportional navigation steering for use as a Doppler frequency reference. To accom law, discussed below, the guidance computer gener plish this, the illuminator antenna combines a narrow ates steering commands that ideally keep the missile beam directed toward the target with a broad refer on a collision course with the target. ence beam that encompasses the missile throughout Semiactive, home-all-the-way guidance is used in flight (Fig. 2). This type of guidance is called "semi many missile systems throughout the world. Its pri active" because the transmitter and receiver, al mary advantage is that it provides moderately good though linked, are not colocated. area defense coverage, i.e., intercept range, by virtue A semiactive missile must also be able to home pas of high power ship-based or ground-based illumina sively on electronic countermeasures signals radiated tion combined with up-and-over flight trajectories. by the target. Such signals may be used to screen the (Such trajectories are achieved by launching the mis target-reflected signal or otherwise deceive the semi sile at a relatively steep angle and allowing it to climb active receiver. Home-on-jam processing is automati above the target early in flight so as to better main cally selected when the target skin return cannot be tain available kinematic energy.) One of the major identified and when incoherent energy is present limitations of home-all-the-way guidance is its rela whose frequency and angle of arrival are approxi tively low firepower, since one of only a few illumi mately correct. nators must be committed to a single target for the entire missile flight. A second limitation is illustrated Home-All-the-Way Guidance in Fig. 4, which shows the many signals that may be A home-all-the-way guidance ' policy is one in present at the missile's receiving antenna. Clearly, a which the missile derives its own steering commands missile receiver that must pick out the target signal throughout flight based on the processing of signals immediately after launch has a much more difficult received from the target. Such a policy requires that task than it would if acquisition could be delayed un the target be acquired either prior to or shortly after til later in flight. launch. Figure 3 illustrates the various phases in the firing of an SM-l, which uses home-all-the-way guid Midcourse Plus Terminal Guidance ance. On the launcher, the missile is provided with a Midcourse guidance implies an intermediate flight post-launch prediction of where it should look for phase in which an external source supplies sufficient the target in angle and Doppler frequency. Following information to allow the missile to guide or steer an unguided boost phase, the missile seeker acquires toward the target without having to home. Two types Missile Guidance System Figure 2 - Semiactive guidance concept. Both SM-1 and SM-2 em ploy continuous wave, semiactive guidance. The target is illumi Steering commands~-.:.-~ nated by highly pure, sinusoidal to autopilot RF energy that allows objects to be distinguished on the basis of their velocity by the Doppler shift of the reflected energy. Homing is semiactive in the sense that the transmitter, in this case a high power shipborne radar, and the missile receiver are linked but not colocated. A passive homing ca pability is also provided to cope with possible jamming by the tar get. The combination of the on board homing receiver and the steerable front antenna is known as a "seeker." In addition to target tracking, the seeker supplies in formation to the guidance compu ter for computing missile steering commands. 290 Johns Hopkins APL Technical Digest Endgame efficiently to engage a larger number of targets. If the • Target detection by fuze • Warhead detonation missile has the kinematic capability and if the mid course trajectory control is sufficiently accurate, then -C.,r\'/,:)) a small target can be engaged at a longer range than ~-Y~' would be possible if seeker acquisition were required Homing Phase early in flight, as in the home-all-the-way case. • Missile homes to target • Semiactive or home-on-jam The shorter missile-to-target propagation distance guidance at the start of the terminal homing phase also tends to enhance the target return relative to some of the sources of interference shown in Fig. 4. In particular, Midcourse Phase (SM-2 only) the missile has a much better chance of "seeing" the • Missile flies to vicinity of target target in the presence of standoff jamming following • Guidance intelligence supplied by ship a period of midcourse guidance than it would imme diately after launch. L Boost Phase 1;'~ ( • Missile achieves supersonic speed ~ • Unguided flight STANDARD MISSILE-2 Initial ization MIDCOURSE GUIDANCE • Missile on launcher • Initialization data supplied by ship Midcourse guidance has been implemented in SM-2 by adding to SM-I an inertial reference unit together Figure 3 - STANDARD Missile operational sequence. On the launcher, the SM-1 is supplied with target acquisition with missile-ship communication links. The inertial data and begins homing shortly after the unguided boost reference unit consists of an instrument package cou phase of flight. The SM-2, however, undergoes a midcourse pled with a special-purpose computer. The instru phase under ship control; at a predetermined point much ments include three accelerometers, two rate-inte later in flight, target acquisition occurs and homing begins. In the endgame phase, the target is detected by a missile grating gyros for measuring angular motion perpen borne proximity fuze and the warhead is detonated close to dicular to the missile body, and a space-stable, single the target. axis platform for monitoring missile roll. The com puter solves the equations of motion to keep track of missile position, velocity, and attitude in the inertial of midcourse guidance are commonly used - com coordinate frame established prior to launch. mand and inertial. In a command system, missile steering signals are directly supplied in an agreed upon coordinate frame. In an inertial system, the AEGIS Command Midcourse Guidance missile guides toward a point or a succession of In the AEGIS Combat System, SM-2 is command points in inertial space based on externally supplied guided during midcourse flight under direct control information. of the ship (Fig. Sa). The AN/ SPY-IA radar performs The use of midcourse guidance followed by a rela missile and target tracking and also serves as the ship tively short period of terminal homing offers a signi board data link transceiver. Steering commands are ficant improvement in firepower and missile inter computed by the ship weapon-control computer and cept coverage ;if the ship system can provide the nec transmitted on the uplink to the missile, which ac essary support. Firepower is clearly increased over knowledges receipt via the downlink. The midcourse home-all-the-way guidance because semiactive hom guidance law is a form of proportional navigation ing is restricted to the terminal portion of flight. with appropriate trajectory-shaping modifications. Thus, the shipboard illuminators can be used more The inertial reference unit is used primarily as an atti- Jamming / (/ ~ Figure 4 - Typical signals received by the missile in flight. Good steering information can usually be derived----- from either target-reflected energy or jamming energy from the target. Interfering signals, which can exceed the target skin return by many orders of magnitude, must be rejected. Interference derives from natural and man-made environments, both friendly and unfriendly. Volume 2, Number 4, 1981 291 (a) AEGIS/SM·2 trajectory control Uplink commands and Terminal illumination Figure 5 - SM·2 midcourse plus downl;nk "knoy:...~_____ I semiactive terminal guidance. In -----~= ...... the AEGIS application (a), SM·2 is guided by the ship through the / ~--~::;;~ AN/SPY-1A midcourse phase of flight. Uplink target tracking acceleration commands, which are acknowledged via the down .-1 link, are converted into steering signals, by the missile inertial ref erence unit. Terminal homing is commanded by uplink and can be delayed longer than in TERRIER or TARTAR because of greater mid (b) TERRIER·TARTAR/SM·2 trajectory control course accuracy. Terminal illumination In TERRIER and TARTAR Combat I Tracking radar Search radar Systems (b), the SM·2 uses its iner tial reference unit to guide itself to a point in space. New points are sent to the missile by the ship via uplink if course changes are re quired. Missile flight progress is ~~~~~, monitored by means of downlink information. The missile switches to terminal homing at a predeter mined time prior to intercept, sub ject to uplink revision. tude reference system, transforming the uplink guid As in AEGIS, at a predetermined time in the flight, ance commands in inertial coordinates to steering the weapon system assigns an illuminator to support commands in missile body coordinates. At a prede the terminal homing phase. Doppler frequency and termined time prior to intercept, the weapon system angle information required for target acquisition by assigns an illuminator to the engagement to support the missile seeker are computed from inertial refer the terminal phase. Shortly thereafter, the missile is ence unit and uplink data. Terminal hand over , which told to search for the target based on updated seeker follows acquisition, cannot be delayed as long as in pointing and Doppler frequency information. The AEGIS because of greater uncertainties in missile po change from midcourse to terminal guidance, known sition and heading. Intercept coverage of TERRIER as "handover," occurs following target acquisition and TARTAR ships with SM-2 is also substantially in by the missile seeker. The improvement in intercept creased relative to SM-I against both incoming and capability relative to SM-I is substantial in both crossing targets. downrange and cross range because of the use of mid course guidance. APL Contributions TERRIER and TARTAR Initially, APL contributed significantly to the de velopment of SM-2 midcourse guidance in the TER Inertial Midcourse Guidance RIER and TARTAR application by formulating the In the TERRIER and TARTAR application, SM-2 initial requirements and defining many of the basic uses inertial midcourse guidance for self-navigation, concepts. This was a challenging undertaking, for command guidance being impractical because these not only was it to be the first tactical missile to use in combat systems do not track the missile. Instead, us ertial midcourse guidance, but it had to be compati ing target data derived from a three-dimensional ble with two existing shipboard systems. For a time, search radar, these ship systems direct the missile to a this work proceded in parallel with the development point in space, or to a succession of points in case the of the new AEGIS Combat System. When the advan target changes course and/or speed (Fig. 5b). New tages of a common missile became apparent, APL guidance points, along with target position and ve helped define the resulting configuration through locity data, are communicated via the uplink as re participation on a number of steering groups char quired. Missile information is downlinked, permit tered by the Naval Sea Systems Command. ting the ship system to monitor flight progress. The Subsequently, APL played a major role in connec midcourse guidance law is known as explicit guidance tion with the development of both the AEGIS and because the steering equations are explicit functions TERRIER data links. From 1973 to 1976, extensive of the current and desired boundary conditions (posi AEGIS link-compatibility tests were periodically con tion and velocity). The missile's knowledge of its own ducted with APL participation. Concurrently, the position and velocity is, of course, provided by the Naval Sea Systems Command requested that APL de inertial reference unit. sign and evaluate an alternative AEGIS uplink re- 292 Johns Hopkins APL Technical Digest ceiver / decoder using improved signal processing con ceived from the target (most importantly, angle of ar cepts. The resulting design, successfully tested in rival) vary with target and missile motion. The seeker 1977, was shown to be highly tolerant of expected antenna receiving the signal is inertially stabilized to variations in the uplink signal waveform. Subse remove missile pitch and yaw motion. The missile re quently, many of the features of the design were in ceiver has two primary outputs, a line-of-sight angle corporated in the production version of the receiver / measurement and a closing velocity (Doppler) mea decoder. In addition, APL guided the development of surement. The guidance computer uses these mea the TERRIER and TARTAR uplink and downlink sys surements to generate steering commands in accord tems, performing much of the preliminary design ance with a proportional navigation guidance law. In work for the downlink system. Test and evaluation the same manner as during the midcourse guidance equipment for these systems was also designed and phase, the autopilot converts the guidance compu built at the Laboratory, culminating in final system ter's electronic commands to missile tail deflections integration testing at APL in 1976 prior to flight so as to alter the missile's trajectory. The result is testing at sea. lateral missile motion. With longitudinal motion pro vided by propulsion, the missile heads toward an in STANDARD MISSILE-2 tercept with the target. TERMINAL GUIDANCE The final phase of flight, known as the endgame, is characterized by a high-speed encounter with the "Terminal guidance" refers to the self-navigation target. Accurate timing of the warhead detonation is phase following midcourse guidance during which essential in this phase. When the missile and the SM-2 homes on the target until intercept occurs and target are close, a small radar aboard the missile, the missile warhead is detonated. A terminal guid known as a target detection device or proximity fuze, ance mode is necessary because midcourse guidance senses the target's presence. A calculation is then per is not sufficiently accurate, even at short range, to formed to determine the optimum time to trigger consistently achieve miss distances less than the lethal warhead detonation. If the missile collides with the radius of the warhead. The same terminal guidance target before that time, a contact fuze detonates the equations are used in all SM-2 missile variants. warhead upon impact. Many factors affect the success or failure of the missile during the terminal guidance phase. At the desired time of handover, seeker acquisition is the MONOPULSE RECEIVER DEVELOPMENT primary consideration. As intercept is approached, As previously indicated, the purpose of the missile the kinematic capability of the missile (i.e., speed and homing receiver is to provide an accurate measure maneuverability) relative to that of the target is criti ment of the angle of arrival of the target signal with cal. As might be expected, crossing and/ or maneu respect to the boresight axis of the seeker antenna. vering targets tend to be more stressing than directly Three basic methods are commonly used in missile incoming, nonmaneuvering threats. Other factors receivers to measure angle error: scan, interferome having a major effect on guidance accuracy are an ter, and monopulse 1 processing. As explained below, angle scintillation phenomenon known as target glint scanning receivers induce an amplitude variation on and, occasionally, severe fades in the target-reflected the received signal in order to derive a measurement signal at a critical time prior to intercept. The extent of angle error. On the other hand, an interferometer to which the missile is not steering toward the actual measures the phase difference in the return signal as intercept point at handover, i.e., the missile heading received at different antenna ports to obtain an esti error, can also affect the final miss distance. mate of the off-boresight angle. Monopulse receivers Figure 6 is a block diagram of the SM-2 terminal can be designed to make use of either amplitude or guidance loop. The characteristics of the signal re- phase differences depending on the type of antenna Propulsion (longitudinal) Angle of ! Missile Tail arrival acceleration deflections Missile receiver Guidance Missile Target-reflected signal Seeker (electronic counter- computer commands (lateral) Autopilot airframe/ (movable antenna, !'"-'+' cou ntermeasu res (proportional or jamming aerodynamics inertially stabilized) features) navigation) Missile- target relative Missile velocity motion Figure 6 - Terminal guidance functional block diagram_ STANDARD Missile terminal guidance is accomplished by proces sing an RF signal received from the target The receiver determines the angle and relative velocity of the target while the guidance computer translates this information into steering commands_ The autopilot adjusts the aerodynamic control sur faces to alter the missile's trajectory as required_ Kinematic energy is provided by a solid rocket propulsion system. Vo lume 2, Number 4, 1981 293 PROPORTIONAL target maneuvers. This is achieved by wave-illumination homing system, a NAVIGATION making the rate of change of the mis good estimate of the closing rate can sile heading directly proportional to frequently be derived from the Dop The problem of guiding a missile the rate of rotation of the line of pler frequency of the target return. to an eventual near miss or collision sight. Ideally, this implies that the The angular rate of li ne-of-sight with a target using only the observed missile respond instantaneously to rotation, a, is measured by the mis motion of the missile-target line of changes in the line-of-sight direction. sile seeker. The seeker antenna as sight has a number of potential solu In reality, there is some delay in re sembly, which is stabilized with re tions. Probably the most obvious is a turning to a constant bearing trajec spect to missile body motion, tracks simple pursuit course, in which the tory following a target maneuver be the target. The tracking error, E/1/ ' missile always flies directly toward cause of finite missile responsiveness derived from the signal processor, the current target location. The dis and filtering that is introduced to together with the antenna rotation advantage of this approach is the reduce the effects of noise. rate, ap ' measured by antenna stringency of the missile maneuvers In mathematical terms, the pro mounted gyros, provides the infor that are generally required as inter portional navigation steering law is mation to compute the line-of-sight cept is approached. A more favor given by rate; i.e., able trajectory is a constant bearing course in which the missile leads the 'Y M = Na, (1) target, similar to the manner in a = ap + E/I/ (2) which a projectile is traditionally where 'Y M is the rate of change of fired at a moving object. If the mis missile heading, a is the rate of rota Computation of steering commands sile flies a course that keeps the rela tion of the line of sight, and N is a is performed by the missile guidance tive missile-target velocity aligned parameter known as the navigation computer according to the above re with the line of sight, a collision will ratio. lationships. In addition to noise fil always occur. If missile speed and The navigation ratio governs the ters, compensations are usually in target speed and course are constant, responsiveness of the missile in cor troduced to account for such factors the missile trajectory is a straight line recting for line-of-sight rotation. as variations in missile velocity and to the intercept point, as is shown in Theory indicates that its value should radome effects. Steering command the illustration. be proportional to the missile-target limits are also imposed so that the Proportional navigation is a steer closing rate, i.e., the higher the clos missile maintains aerodynamic sta ing law that is intended to yield a ing rate, the more responsive the mis bility and does not exceed its struc constant bearing course even if the sile must be. With a continuous- tural capability. Homing geometry Intercept -.c!~../'"I • ~~J ------~... ~~ ./ Basic relationships: 1. Steering equation for proportional navigation is "Y M = N G. 2. From geometry, a = fT + Gp . 3. To obtain a measure of the signal processor provides f m ("'" f ) cr, T and seeker-mounted rate gyros provide ap' employed. Phase-comparison monopulse receivers to the target off-boresight angle, while the phase is have become by far the most popular choice in mod directly related to the polar orientation of the target. ern missile receiver applications based on perform This information can be extracted by the receiver by ance and hardware implementation considerations. processing the first harmonic of the scan sideband Until recently, scanning techniques were most frequency. (In the frequency domain, the amplitude common as a result of their relative simplicity. A modulation of the received signal caused by the nu scanning receiver measures angle by either mechani tating antenna pattern is evidenced by sidebands dis cally or electronically nutating the antenna to form a placed from the primary return signal at multiples or conical pattern about the seeker axis (Fig. 7). The harmonics of the scan frequency.) magnitude of the resulting amplitude modulation on One of the major disadvantages of scan systems the received signal can be shown to be proportional has been their inherent susceptibility to amplitUde 294 j ohns Hopkins A PL Technical Digest Q) Q) 't:l 't:l .€ .€ c. c. Target «E «E Figure 7 - Scanning receiver op eration. Nutation of the scanning Radar frequency Video frequency seeker antenna introduces an in tentional amplitude modulation on the received signal. The modu lation causes sidebands about each return, whether from the tar Scanning get or from clutter. Seeker angle seeker error information is derived from the magnitude of the first scan sideband associated with the tar get signal. Scan sidebands asso ciated with clutter can interfere with target detection and track ing. Reference signal antenna ~-~.A modulation. While more modern scanning receivers priate lobes of the subdivided front seeker antenna. have been developed using electronic techniques that The directivity pattern for the SM-2 antenna consists are relatively immune, scan systems are basically at a of four parallel lobes, as shown in Fig. 8. By sum disadvantage because they make a time-sequential ming and differencing combinations of these four determination of angle rather than a simultaneous lobes, the E, Llazimuth ' and Llelevation signals are formed. measurement. A second disadvantage of scan pro Although these three signals contain all the necessary cessing is that the generation of scan sidebands can information for computing angle errors, they are not cause interference in the receiver. This is illustrated in in a useful form and must be amplified, filtered, nor Fig. 7, in which the scan sidebands of clutter overlap malized, and translated to a lower frequency ("vid those of the desired target even though the funda eo") before final processing can be performed. In mental signals are separated in Doppler frequency. many respects, these operations are similar to those Both monopulse and interferometer receivers avoid performed in ordinary radio sets. these disadvantages. Following the microwave arithmetic, the informa tion needed for computing the angle error is con STANDARD Missile-2 Receiver Objectives tained in the amplitude and sign of each of the Ll A primary objective in designing the SM-2 receiver channel signals relative to the E signal. Although sep was to take advantage of available technology to ob arate channels might conceivably have been main tain the high degree of angle measurement accuracy tained for each of these signals until the final angle afforded by one of the simultaneous signal process error computation was performed, the approach ing techniques. Of course, it was recognized that this taken was to combine the E signal with each of the Ll alone would not guarantee the desired level of perfor signals in phase quadrature. Since the Ll signals are mance in all of the signal environments considered typically small compared to the E signal, the phase possible for SM-2 operation. Consequently, a number quadrature combination is approximately equal in of additional key features were defined at the outset amplitude to the E signal and has a phase shift to guide the design of the receiver and associated relative to the E signal that is proportional to the logic. As testing proceeded in conjunction with devel angle error. The new signals are multiplexed through opment, problems were found that invariably were a common limiting amplifier that normalizes the am traced back to a violation of the original basic princi plitudes but preserves the phase relationships. Fol ples. The performance of the final configuration, as lowing normalization, the signals are demultiplexed, demonstrated by extensive ground-based and flight filtered, and phase compared to reproduce the de tests, has since confirmed the soundness of the design sired angle errors for use in pointing the seeker and approach. A brief discussion follows of the basic op providing missile guidance information. This imple eration of the receiver, divided into angle error pro mentation requires less hardware than the separate cessing and Doppler processing functions. channel approach, and also facilitates the preserva tion of relative gain and phase between the E and Ll Angle Error Measurement signals. SM-2 employs phase-comparison monopulse angle processing, enabling target direction to be deter Doppler Processing mined on an instantaneous basis in two angular co When several competing signals are received, the ordinates by comparing the signals received in appro- one with the largest amplitude will inherently be pre- Volume 2, N umber4, 1981 295 pillover Q) Q) .e1:1 .e1:1 c. Target c. «E «E Figure 8 - Monopulse receiver operation. Angle error information Radar frequency is contained in the relative magni tudes of the E and Ll signals formed by summing and differenc Tracking/ ing the signals received by the steering four symmetrically offset lobes of Video signals signal the monopulse antenna. This si processor multaneous measurement ap proach circumvents the amplitude antenna modulation in time-sequential measurement (scanning) re ceivers, resulting in a significant reduction in Doppler-band inter ference. Reference signal antenna ~-----~"i.X I---~ ferred. Therefore, in most signal environments, the eration with General Dynamics/ Pomona personnel desired target signal must be isolated from unwanted for achieving the specified performance, performing signals that may be many orders of magnitude larger. trade-off studies where alternative approaches were In a continuous-wave semiactive system, this is usual apparent, and conducting a comprehensive test pro ly accomplished by taking advantage of the fact that gram on the final configuration. Some early APL the various signals received will possess different contributions to the design were made with the aid of Doppler frequencies because of velocity differences a simulator that greatly facilitated the optimizing of (Fig. 8). Spillover, the signal resulting from energy circuit parameters over a variety of threat environ that enters both the front and reference receivers via ments. Subsequently, the first closed-loop homing the direct path from the illuminator to the missile, tests with actual guidance hardware were made at will have a zero Doppler frequency. Chaff and clutter APL. Throughout the development program, strict have near zero velocity relative to the illuminator, so adherence was paid to the fundamental principles de their Doppler frequency is primarily dependent upon fined at the outset despite occasional pressures to do the missile velocity. Relative to stationary objects, otherwise. The resulting performance, as demon the Doppler frequency of an incoming target will be strated by nearly 100 flight tests, has confirmed both higher, that of a crossing target will be approximately the soundness of the approach and the initial predic the same, and that of an outgoing target will be tions of the APL and General Dynamics/ Pomona lower. Although it is not shown, inherent receiver designers. noise is present throughout the Doppler spectrum. In addition, jamming may occur selectively or at all fre TEST AND EV ALUA TION quencies. The SM-2 guidance system was subjected to an ex To acquire a desired target signal, a technique tensive series of ground-based and flight tests similar to that used in tuning an FM radio receiver is throughout its development, with significant APL used. This consists of searching the spectrum in the participation. In retrospect, the Naval Sea Systems neighborhood of the desired signal until it is located. Command's "build-a-little, test-a-little" philosophy At that time, automatic frequency control is used to was crucial to the success of the development pro maintain frequency tracking. In the missile receiver, cess. As soon as guidance hardware was made avail the search is programmed and the switch to frequen able to APL, emphasis shifted from the simulator to cy tracking is automatic, based on the results of a testing in the APL Guidance System Evaluation Lab signal bandwidth, or coherency, test. A comparison oratory. Subsequently, flight tests were conducted, of the spectra in Figs. 7 and 8 indicates that tracking including instrumented test-range firings and tactical the target signal with a monopulse receiver in a system evaluations at sea. dynamic situation should be easier because it is not contaminated with scan sidebands. Guidance System Evaluation Laboratory The Guidance System Evaluation Laboratory APL Contributions (GSEL) has been used continuously for testing guid APL contributed to the design and development of ance hardware since it was established in 1965. Up the SM-2 monopulse receiver by defining the primary graded substantially in 1980 to better support SM-2 requirements, formulating techniques in close coop- development, the facility consists of an anechoic test 296 Johns Hopkins A PL Technical Digest chamber that houses the guidance section to be After completion of the engineering development tested, generating equipment to reproduce electro flight test phase at White Sands, combat system tests magnetic signal environments representative of ac were conducted in an operational shipboard environ tual flights, and a hybrid computer system to control ment. From a missile standpoint, at-sea testing en test conditions and simulate the dynamic interactions ables modes of operation to be evaluated that are be of the missile and the target. Tests can be performed yond the capabilities of White Sands, partly because to simulate a wide variety of threat situations, in of range safety considerations. AEGIS at-sea testing cluding those involving countermeasures. 2 was conducted in USS NORTON SOUND (AVM-I) in SM-2 testing in GSEL began in 1975 with an evalua 1976, 1977, and 1978; TERRIER testing was con tion of a prototype SM-2 guidance section. Follow-on ducted in USS WAINWRIGHT (CG-28) in 1976 and in evaluations were performed using both preproduc USS MAHAN (DDG-42) in 1978 and 1979. Preceding tion and production units. The primary objectives of shipboard testing, the TERRIERI SM-2 Combat Sys those tests were to conduct an in-depth evaluation of tem was assembled and tested at the APL Land Based the guidance system design and to determine guid Test Site. After successful completion of this evalua ance accuracy in intercepting both known and pro tion, the equipment was transferred to the firing jected threats. In the early stages of development, the ship. During the initial phases of this test program, first objective tended to dominate as design short APL advisors assisted in the training of Navy person comings were invariably uncovered. Working closely nel. with the missile Design Agent, solutions to each Based on the successful SM-2 flight test program in problem involving either hardware or computer pro USS MAHAN, the TERRIER Combat System with the gram modifications, or both, were developed and SM-2 missile was approved for service use on October verified. After the design was reasonably firm, a 2, 1979, by the Chief of Naval Operations. AEGIS comprehensive series of tests was conducted to assess with SM-2 will become operational when the first guidance accuracy in terms of miss distance. That AEGIS ship, TICONDEROGA (CG-47), is deployed. evaluation included tests involving a broad spectrum of threats. The overall conclusion of the test program STANDARD MISSILE-2 was that the design objectives of SM-2 had been UPGRADE PROGRAM achieved. With the initial version of the SM-2 in production, Another important facet of the GSEL test effort attention has focused on extending its guidance sys was in support of flight test activities. The conditions tem's ability to counter the growing threat of high to be employed in planned flight tests were simulated speed, high-altitude, antiship missiles launched from to provide performance predictions and to ensure aircraft. Taking advantage of recent advances in digi that the tests provided a significant demonstration of tal processing techniques, the SM-2 guidance section capability with a reasonable margin for success. Oc has been provided with a fast Fourier transform digi casionally, unexpected phenomena were encountered tal signal processor. This processor is functionally in a firing with a suspected link to the signal environ equivalent to a set of contiguous analog Doppler fil ment. Missile hardware would then be tested in GSEL ters, each having a small bandwidth. The objective is over a range of environmental conditions bracketing to enhance acquisition and tracking of returns from those of the flight. Such examinations often led to a better understanding of missile operation. Flight Testing The initial flight tests of SM-2 were conducted at the White Sands Missile Range in New Mexico. While White Sands is part of the National Range Sys tem operated by the U. S. Army, the Navy as a tenant has access to the range communication, tracking, and support facilities necessary for missile testing. In ad dition, a weapon control system and launcher have been installed at the Navy installation (USS DESERT SHIP) so that the system-to-missile interface is func tionally equivalent to an actual shipboard system. SM-2 test firings were conducted with a Navy crew supported by civilian contractors (Fig. 9). APL main tains a close working relationship through a field of fice and a resident engineer. In addition, APL heads the Computer Programming Committee that devel ops Weapon Control System computer programs to support individual flight test scenarios. APL is also represented on the Test Coordination Panel and par Figure 9 - Launch of an AEGIS SM-2 flight test round at the ticipates in post-flight analyses. USS DESERT SHIP at White Sands Missile Range. Vo lulI/e 2, N UII/ber 4, 1981 297 targets in the presence of obscuring jamming noise. missile carries its own transmitter), infrared, and APL's most significant contribution to the digital sig wideband home-on-jam. Some of the more attractive nal processor design was the development of a digital multimode combinations include semiactive terminal automatic gain control that provides the processor supplemented by either active or infrared, active ter with the ability to handle signals of widely varying minal supplemented by infrared, and these plus wide amplitudes. band home-on-jam. In addition to signal processing improvements, Currently, several programs have been initiated to missile propulsion has been increased to provide the demonstrate the technology required for alternate or kinematic capability for successful high altitude in supplemental guidance modes. Examples include de tercepts. This, in turn, led to the need for radome ac velopment of high power, active transmitters; multi commodations to compensate for the effects of in band antenna systems; and infrared domes for high creased aerodynamic heating. APL again played a speed flight. Physical integration of multiple sensors key role, in conjunction with General Dynamics/ into a guidance package and definition of multimode Pomona, in defining the required changes. The re guidance system control logic are also being ad sulting improved version of SM-2, designated Block dressed. II, is currently in engineering development. NOTES ADVANCED GUIDANCE POSSIBILITIES IThe word "monopulse" originally referred to a radar system that required The feasibility of multiple guidance modes is cur a single pulse to measure the target off-boresight angle. When applied to a continuous-wave receiver, it implies that angle error is measured by simul rently being examined in an exploratory development taneously comparing the return signal as it is received in various sections context with potential applicability to a future STAN of a subdivided antenna. 2For further information on the Guidance System Evaluation Laboratory, DARD Missile., Homing modes being considered, in see W. M. Gray and R. W. Witte, "Guidance System Evaluation Labora addition to semiactive, include active (in which each tory," Johns Hopkins APL Tech. Dig. 1, 144-147 (1980). 298 Johns HopkinsAPL Technical Digest